Small KBOs with diameters less than ~350 km have densities below that of pure water-ice due to their high porosities. However, an object as large as 2002 UX25 is unlikely to have a porosity of more than ~20 percent because ice is more compressed due to the higher pressure in such a large object. Basically, smaller objects tend to have lower densities, while larger objects tend to have higher densities. The low density of 2002 UX25 and its inability to support a high porosity suggest that a very small fraction of its mass is in the form of rocky material.
Figure 1: Artist’s impression of a Kuiper Belt Object.
Figure 2: Densities of objects in and from the Kuiper Belt. Michael E. Brown (2013).
KBOs larger than ~1,000 km tend to have 70 percent or more of their mass in the form of rocky material. The low density of 2002 UX25 poses a problem for the formation of large KBOs if these objects formed from the merger of smaller objects with rock mass fractions as low as what is inferred for 2002 UX25. For example, Eris is one of the largest objects in the Kuiper Belt. To form an object with the volume of Eris would require ~40 objects the size of 2002 UX25. Nevertheless, such an assembled object, even with the additional compression from its greater mass, is expected to have a density of only ~1.0 g/cm³, much less than the measured density of 2.5 g/cm³ for Eris.
A number of explanations have been presented, but none appear likely. One explanation is that the porosity of 2002 UX25 is severely underestimated. 2002 UX25 could have porosity as high as 50 percent, thereby allowing it to have a higher rock mass fraction. Another possibility is that large KBOs have high densities because they have lost much of their icy mantles through the effects of giant impacts. 2002 UX25 challenges the leading model for the formation of large KBOs. Even if 2002 UX25 turns out to be an oddball, its exceptionally low density cannot simply be ignored.
Michael E. Brown (2013), “The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets”, arXiv:1311.0553 [astro-ph.EP]